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Old 10-13-2015, 09:37 AM   #31
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Just for clarification...

Overdrive in not some mysterious component. It simply means a taller than 1:1 gear ratio.

The venerable Ford C4 transmission has 3 speeds or gears. the gear ratios for the 3 speeds are: First is 2.46:1. This means the input shaft turns 2.46 turns for each turn of the output shaft. Second is 1.46:1 and third is 1:1 or each turn of the input shaft equals one turn of the output shaft. Gears 1 and 2 are "underdriven" as the output shaft turns slower (at any given speed) than the input shaft. Third is considerd a "direct drive" as the shaft speeds are equal.

The Ford E4OD is a 4-speed transmission, with gear ratios of:

1st=2.71
2nd=1.54
3rd=1.00
4th=.71

Like the C4, gears 1 and 2 are 'underdriven' and gear 3 is 'direct'. But! There is a 4th gear who's ratio is 0.71:1 This means the input shaft turns only .71 of a turn for each turn of the output shaft. It spins slower than the output shaft so it is called "overdriven".

This is all that overdrive is. Its just indicates a final gear that is overdriven. It's not some sort of additional component that kicks in and out inside the transmission. Locking out "overdrive" simply means that you are not allowing your transmission to shift into its highest gear. It would be like driving your old C4 and keeping the selector in 2.

There is nothing to "burn out" by running in overdrive. What can happen (and did in early OD transmissions) is that you run the input shaft so slow that you cannot get adequate oil flow for proper cooling. The oil pump is run off the input shaft, and if you are towing a heavy load and not moving enough oil, temps can rise dramatically.

Modern transmissions are perfectly happy towing up to the max weight in OD. However, to aid things, you can select OD off to keep the revs up and the oil flowing. But remember, all that button does is prevent the trans from shifting into its highest gear.

Tim
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Old 10-13-2015, 11:00 AM   #32
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Originally Posted by Wumba View Post
When climbing into hilly or mountainous terrain I use tow mode or drop into the Allison transmission 6 speed. 2006 Chevy 2500HD diesel.
Wumba, what do you mean when you say "I... drop into the Allison transmission 6 speed"? You're using the Allison transmission 6 speed all the time, right?
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Old 10-13-2015, 11:11 AM   #33
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Quote:
Originally Posted by Cowracer View Post
Just for clarification...

Overdrive in not some mysterious component. It simply means a taller than 1:1 gear ratio.

The venerable Ford C4 transmission has 3 speeds or gears. the gear ratios for the 3 speeds are: First is 2.46:1. This means the input shaft turns 2.46 turns for each turn of the output shaft. Second is 1.46:1 and third is 1:1 or each turn of the input shaft equals one turn of the output shaft. Gears 1 and 2 are "underdriven" as the output shaft turns slower (at any given speed) than the input shaft. Third is considerd a "direct drive" as the shaft speeds are equal.

The Ford E4OD is a 4-speed transmission, with gear ratios of:

1st=2.71
2nd=1.54
3rd=1.00
4th=.71

Like the C4, gears 1 and 2 are 'underdriven' and gear 3 is 'direct'. But! There is a 4th gear who's ratio is 0.71:1 This means the input shaft turns only .71 of a turn for each turn of the output shaft. It spins slower than the output shaft so it is called "overdriven".

This is all that overdrive is. Its just indicates a final gear that is overdriven. It's not some sort of additional component that kicks in and out inside the transmission. Locking out "overdrive" simply means that you are not allowing your transmission to shift into its highest gear. It would be like driving your old C4 and keeping the selector in 2.

There is nothing to "burn out" by running in overdrive. What can happen (and did in early OD transmissions) is that you run the input shaft so slow that you cannot get adequate oil flow for proper cooling. The oil pump is run off the input shaft, and if you are towing a heavy load and not moving enough oil, temps can rise dramatically.

Modern transmissions are perfectly happy towing up to the max weight in OD. However, to aid things, you can select OD off to keep the revs up and the oil flowing. But remember, all that button does is prevent the trans from shifting into its highest gear.

Tim
Ecellent explanation, Tim !!

The ratios you show for the Ford E4OD are very close to what shows on my Scan Gauge. I think they are 2.85 for 1st, 1.55 for 2nd, .99 for 3rd, and .69 for OD. With the Scan Gauge, i also monitor my transmission temperature, which stays in the 160 to 180 range while towing in OD on a hot day. I only turn off OD when in hilly areas, and the tranny would be shifting to 3rd. I do not usually run the cruise control, and then only in 3rd.

My owners manual reads something like: "Turn off OD when the transmission frequently shifts to a lower gear".
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